tipc: use bearer_disable to disable bearer in tipc_l2_device_event

As bearer pointer is known in tipc_l2_device_event(), it's unnecessary
to search it again in tipc_disable_bearer(). If tipc_disable_bearer()
is replaced with bearer_disable() in tipc_l2_device_event(), this will
help us save a bit time when bearer is disabled.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Tested-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>